Currently, in the fabrication of titanium and titanium-alloy articles, thermal or heat-treating processes are included in the manufacturing process. These steps are to ensure that material grain size is produced and maintained at a level that is as small as possible. As such, it is the normal practice to employ a full annealing, i.e. recrystallization, or at least stress-relieving heat treatment steps in conjunction with any cold or hot work or forming performed on the material. There have been exhaustive attempts to eliminate these thermal treatment, or heat-treating, manufacturing process steps, which can account for up to approximately 20% of the costs not to mention processing cycle time associated with producing a titanium or titanium-alloy article or fastener, such as a rivet, lockbolt or threaded pin.
The resulting grain size of formed material is critical to both its ductility and strength among other properties. In general, grain sizes larger than or equal to those identified as a number 6 (i.e., less than or equal to a number 5 as defined by ASTM E 112) are not desirable for most mechanical work or forming operations. Currently, grain sizes of formed material typically range from a grain size of 6 to 10. A duplexed grain size, defined as a significant difference in grain size depending upon location, should also be avoided. Grain size is of special importance and generally increases in the degree of importance as the material is mechanically formed or deformed to larger levels. As a rule, the finer the grain, the better the resulting formability. Recent research by Gysler et al. on “Influence of Grain Size on the Ductility of Age-Hardened Titanium Alloys” has documented the directly proportional relationship between smaller grain size and improved material properties in titanium and titanium-alloy materials. It has been shown that smaller grain size improves the strength of the material, increases ductility, and enhances the corrosion resistance.
Currently, in the fabrication of titanium and titanium-alloy articles or components, such as fasteners, additional thermal or heat treatment steps are necessary and included in the manufacturing process. These subsequent steps are intended to counteract or offset the effects on grain size resulting from of the mechanical working or forming imparted to the material during its manufacturing process.
As indicated earlier, grain size is of special importance and generally increases in the degree of importance as the material is formed in progressively higher levels of manufacturing. As a rule, the finer the grain, the better the formability and other associated characteristics. This has been known to be the case in aluminum and aluminum alloys for a long period of time and is consequently well-documented. For titanium and titanium-alloys, not as much work has been done in documenting the relationship between grain size and other material properties.
However, recent research has documented the directly proportional relationship between smaller, i.e., finer, grain size and improved material properties in titanium and titanium-alloy materials. It would therefore be desirable to provide a process for forming titanium and titanium-alloy articles having smaller grain sizes while reducing the number of processing steps.